The Food and Drug Administration said proposed prescription painkillers should fill an unmet medical need for patients who have no other "relatively safer" alternatives, suggesting Merck & Co.'s bid to have a Vioxx-like successor drug approved in the U.S. faces a tough road.

The FDA's view was outlined in a March 21 memo posted on the agency's Web site ahead of a meeting tomorrow of outside medical experts who are being asked to weigh in on whether the agency should approve Arcoxia. The Merck drug falls into the same class as Vioxx, which was widely used to treat arthritis and other painful conditions.

Vioxx was pulled off the market in September 2004 after it was linked to an increased risk of heart attacks and strokes, and in 2005, the FDA asked Pfizer Inc. to pull Bextra off the market. Pfizer was allowed to keep its other so-called Cox-2 painkiller, Celebrex, on the market with warnings.